The present invention relates to a moving catalyst bed reactor for the catalytic treatment of hydrocarbons, in particular catalytic desulphurization and demetallization of petroleum residues.
The invention relates more in particular to a moving catalyst bed reactor comprising a normally vertically extending cylindrical vessel having separate inlets for reactant and catalyst at the upper part of the vessel. The vessel contains a catalyst bed supporting means in the shape of a downwardly converging conical surface, the lower end thereof being connected to a vertical outlet channel with a circumferential wall for the combined passage of catalyst and separation of reactor effluent. The circumferential wall is provided with a circular screen section for withdrawing reactor effluent from catalyst, said vessel further having separate outlets for reactor effluent and catalyst downstream of said screen section.
Moving bed reactors of a design as identified above are well known. An example thereof has been described, e.g. in U.S. Pat. No. 3,883,312. According to this patent both, the used catalyst particles and the reactor effluent, are removed from a moving catalyst bed reactor through the same withdrawal conduit. The reactor effluent exits the conduit through openings of a circumferential screen section in its wall.
In recent years there is a tendency to carry out processes of this kind on a larger scale, which has resulted in problems in the design of reactors when they are scaled-up to large capacities. One such problem is the resulting size of the catalyst bed and the cross-sectional area of the catalyst/reactor effluent outlet channel required to allow catalyst to pass through the outlet channel at an acceptable rate. Increase of the cross-sectional area of the catalyst/reactor outlet channel however, means impairment of the catalyst/reactor effluent separation efficiency in that separation of reactor effluent from central parts of the bed becomes more and more difficult.
U.S. Pat. No. 4,357,304 relates to an improved moving catalyst bed reactor of a design in which the above scaling-up difficulty and the increase in the size of the outlet channel has been overcome. According to this patent the catalyst/reactor effluent outlet channel is provided with further screen sections within its wall for withdrawal of reactor effluent from catalyst.
A further problem associated with upscaling of catalytic treatment processes resides in the constructional design of large capacity reactors. Increase of the diameter of the catalyst/reactor effluent outlet channel, and thus increase of the circumferential screen section in the wall of the channel complicates manufacture and installation of the screen section. Another consequence of process upscaling is the fact that the screen sections in the outlet channels should be able to withstand ever increasing loads both in horizontal and in vertical direction.
In co-pending application Ser. No. 390,452, assigned to the same assignee as this application, a moving catalyst bed reactor is described, in which reactor the circumferential screen section of a catalyst/reactor effluent outlet channel is composed of a plurality of separate screen segments which are preferably supported in lateral outward direction and in vertical direction by special supporting elements. The separate screen segments enable a simple fabrication and installation of the complete screen section in a reactor. According to the above-mentioned publications, the catalyst/reactor effluent outlet channels, of especially large reactor vessels, may be provided with further screen sections within the outlet channels for promoting reactor effluent separation from the catalyst particles passing through said channels.
Especially in very large reactors, supporting of the screen sections in the catalyst/reactor effluent outlet channels may form a severe constructional problem, in particular if the channels are to be provided with extra internal screen sections within the outlet channels to meet the required separation efficiency.
If a reactor is provided with several superimposed catalyst beds, for example required for allowing intermediate cooling of reactor effluent, requirements are set as to the redistribution of fluid from a catalyst bed over a next lower catalyst bed. In particular, if large diameter reactors are to be applied, it is very important to have a proper redistribution of fluid over the catalyst bed to keep the catalyst bed height within acceptable limits. Increase of the bed height means increase of the length of the reactor vessel and thus a considerable rise in manufacturing costs in particular if the vessel is a so-called pressure vessel, as in normal in catalytic treatment processes.
As already mentioned in the above, high capacity operation requires in general the application of further screen sections for reactor effluent withdrawal in the known moving catalyst bed reactors. For supporting such further internal screen sections special supporting constructions will be necessary for transferring the load on these screen sections to the wall of the outlet channel in which these sections are arranged.